Activity Summary
Students conduct an inquiry into the meaning of m in E =
mc2 by exploring how objects of mass interact during a chemical
reaction in a plastic bag, and by measuring mass before and after chemicals are
mixed together.

Learning Objectives
Students will be able to:

explain what the m in E = mc2 represents.

relate that mass in a chemical reaction is always conserved.

convey that atoms rearrange themselves in chemical reactions to form
different molecules and compounds.

Background
Without
the insights of scientists before him, Albert Einstein would not have had the
foundation from which to make his brilliant leap of understanding about the
equivalence of mass and energy. Two of the people responsible for providing a
critical understanding about mass were Antoine-Laurent Lavoisier and his wife,
Marie Anne. By day, Lavoisier was a tax collector. But his true passion was
chemistry. A meticulous experimenter, Lavoisier was the first to demonstrate
that matter is conserved in a chemical reaction. In the late 1700s, Lavoisier
showed that when water was turned to steam, nothing was lost—the water
was just transformed. His wife provided detailed drawings of his experiments
and translations of other scientists' work.

In this activity, students examine the components of a chemical reaction and
make measurements to confirm the conservation of mass in a closed system (quart
freezer bag). Students will gain an understanding of the meaning of mass in a
reaction. They will learn how objects of mass can interact and change and that
mass is always conserved in a chemical reaction.

The reaction in this activity is between two fairly harmless chemicals-citric
acid (H3C6H5O7) and baking soda
(NaHCO3), both white powders. The reaction can only occur in the
presence of water. The reaction produces a gas (CO2) and the
compound sodium citrate
(Na3C6H5O7). The balanced reaction
looks like this:

H3C6H5O7 + 3NaHCO3 ⇒
Na3C6H5O7 + 3H2O +
3CO2

The reaction is endothermic so students will feel the plastic bag get
colder when the three ingredients are mixed. To further verify for students
that a chemical reaction is taking place, you may want to add an acid-base
indicator, such as phenol red, to reveal that a change (in pH) has occurred
after the reaction has taken place.

Key Terms

chemical
reaction: A process in which one or more substances are changed into other
substances.

conservation of mass: A law stating that the products of a chemical
reaction always have the same total mass as that of the reactants.

endothermic: Chemical reactions that take in heat from their
surroundings.

exothermic: Chemical reactions that give off heat to their
surroundings.

mass: The amount of matter an object contains.

products: Substances resulting from a chemical reaction.

reactants: Substances that take part in a chemical reaction.

weight: The force of gravity acting on matter.

Obtain the necessary chemicals from your chemistry lab, a science supply
house, or stores in your area. Baking soda is sold in grocery stores and citric
acid, which is used in canning and winemaking, is sold in some drug stores.

Begin by pointing out the m in E = mc2 and asking
what this letter stands for. Students may say, "How much something weighs."
Pick up an object such as a stapler and ask how much the stapler would weigh
independent of a gravitational field. The answer is "nothing" because weight is
just a name for the force of gravitational attraction that exists between two
objects, in this case Earth and the stapler. The fact that weight is simply a
force of attraction, not a unit of mass, can be a difficult concept for some
students. Try to help students understand that mass is the amount of "stuff,"
or atoms, an object contains. Mass is defined by units like grams and
kilograms.

To help students start thinking about mass, conduct a demonstration about
how mass interacts.
Have
one 250 milliliter beaker filled with 100 grams of citric acid and another
filled with 100 grams of baking soda. Hold the two cups out in front of you and
tell students the cups contain two different chemicals of equal mass. Ask
students to predict what will happen if the two of them are mixed together.
After students answer, mix the substances. (Nothing will happen.) Ask students
why nothing occurred. What evidence did they have that led them to their
conclusion? (No noise, no smoke, no visual changes seen, etc.)

Now that students understand that not all mass reacts, have them conduct
their activity in which a chemical reaction does occur. Organize students into
teams and give each team a copy of the student handout and other materials.

Review safety rules with students:

Wear goggles.

Do not allow the chemicals to touch bare skin. (If students accidentally
touch a substance, tell them to wash their hands immediately. These chemicals
are about as dangerous as lemon juice, but precautions should be followed.)

When chemicals are mixed in the plastic bag, make sure the opening of the
bag is well sealed and pointed away from students.

Students may feel the bag while the reaction is proceeding, but they
should not squeeze it.

Circulate around the room as student teams do the experiment. Help
anyone having trouble using the balance.

When students are finished making their observations, collect the
bags in a bucket or container. Give students time to answer the questions on
their student handout. Then have a discussion about the nature of a chemical
reaction. Why was there a reaction in the student activity but not one in the
teacher demonstration? (The water in the student activity served as a solvent
that allowed the two solids to react.) How was mass conserved in this reaction?
(Although a chemical change occurred—two dry, white granular solids when
mixed with water became a gas and a dissolved solid in a water
medium—mass was conserved as evidenced by the almost identical masses
before and after the reaction.) Brainstorm with students some possible reasons
for any mass differences they saw. (Small weight differences, on order of 0.1
to 0.3 percent, may be seen; see Activity Answer for more information.)

As an extension, turn the investigation into a quantitative one and
have students measure the temperature of the reaction as the reaction proceeds
(a thermometer should be placed on a desk, the bag set down on the bulb end of
the thermometer, and the temperature read every minute or so). Then, have students
change the quantities of one of the checmicals and take more temperature data. Plot
both sets of data on a single set of axes. Ask students interpret their results.

It
is important to stress that mass is always conserved in a chemical reaction in
a closed system. (An extremely small amount of mass—on order of a few
parts per trillion—is lost or gained when light and/or heat is absorbed
or released in a reaction. But for all practical purposes this is too small to
measure.)

The reaction in a quart bag is a good example of a closed system. However, any
measurement
contains a degree of uncertainty. There may be a slight difference in mass due
to loss of gas or errors in measurement. (Sample test results showed a 0.1
percent to 0.3 percent weight difference.) In an experiment in an open system,
such as weighing a piece of wood, burning it, and weighing the ashes
afterwards, it would appear that mass is not conserved. But in fact it is. It
is just that the escape of invisible gases, both carbon dioxide and water,
prevent the measurement of the mass of all of the products.

The word mass is used deliberately in the student activity instead of the more
commonly used weight. If necessary, reinforce the difference between the two
terms when students use their balances to weigh their bags.

Student Handout Questions

When a reaction gives off heat, it is called exothermic.When a reaction
absorbs heat, it is called endothermic. Is the reaction you observed
endothermic or exothermic? The reaction is endothermic. Students should feel
the bag getting colder as the reaction proceeds. Exothermic reactions are much
more familiar to students (striking a match, burning a candle) than endothermic
ones. Students may need help in understanding that the chemicals in the bag
cool because heat is being used in the reaction to turn the reactants into
products.

How did the mass of the reactants compare to the mass of the products? Use
this formula to calculate any percent difference in the mass between the
two:

absolute value of the difference between the two masses divided by mass before
reaction x 100 = percent difference

Students should arrive at the conclusion that the mass of the products is
extremely close to the mass of the reactants.

What might have caused any difference in mass that you found? There may
be a small difference in mass due to loss of gas from the plastic bag or errors
in measurement. Accept reasonable answers.

What evidence would you give to show that a chemical reaction did indeed
occur? There are two signs that a chemical reaction occurred: a change in
temperature and the formation of a gas.

Web Sites

NOVA—Einstein's Big Ideawww.pbs.org/nova/einstein
Hear top physicists explain E = mc2, discover the legacy of
the equation, see how much energy matter contains, learn how today's physicists
are working with the equation, read quotes from Einstein, and more on this
companion Web site.

Periodic Table of the Elementsperiodic.lanl.gov
Provides a periodic table and information about each element.

Physics: The Human Adventure: From Copernicus to Einstein and Beyond
by Gerald James Holton and Stephen G. Brush. Rutgers University Press, 2001.
Introduces concepts and theories in physical science and features a chapter on
the conservation of mass.

The Visual Dictionary of Chemistry
by Jack Challoner. DK Publishing, 1996.
Provides photographs of chemical experiments, illustrations of molecules and
chemical reactions, and information about the periodic table of elements.

The "Messing With Mass" activity aligns with the following National Science
Education Standards (see books.nap.edu/html/nses).

Grades 5-8
Science Standard
Physical Science

Properties and changes of properties in matter

Transfer of energy

Grades 9-12
Science Standard
Physical Science

Chemical reactions

Classroom Activity Author

Jeff
Lockwood taught high school astronomy, physics, and Earth science for 28 years.
He has authored numerous curriculum projects and has provided instruction on
curriculum development and science teaching methods for more than a decade.

Einstein's Big Idea

Original broadcast:October 11, 2005

Major funding for NOVA is provided by Google. Additional funding is provided by
the Corporation for Public Broadcasting, and public television viewers.

Major funding for Einstein's Big Idea is provided by the National Science
Foundation. Additional funding is provided by the Alfred P. Sloan Foundation,
and the U.S. Department of Energy.

This material is based upon work supported by the National Science Foundation
under Grant No. 0407104. Any opinions, findings, and conclusions or
recommendations expressed in this material are those of the author(s) and do
not necessarily reflect the views of the National Science Foundation.